Ultrasound imaging systems have become an important diagnostic tool in many medical specialties. One important advantage of an ultrasound imaging system is real-time scanning. For example, an ultrasound imaging system can produce images so rapidly that a sonographer can scan internal organs or can discern motion within a body, such as blood flow, with real-time, interactive, visual feedback. This allows the sonographer to examine structures of interest and to modify the examination in real-time, thereby improving both diagnostic quality and patient throughput.
Along with the advantages of real-time, interactive, visual feedback, sonographers are still concerned with system resolution. In an ultrasound imaging system, system resolution depends on the system's ability to focus. The ability to focus depends, in turn, on the effective aperture of a transducer element array in a probe associated with the ultrasound imaging system. Currently two types of arrangements of transducer array elements are used for real-time, ultrasound imaging systems.
One arrangement comprises a single transducer element or an annular array of transducer elements. Ultrasound imaging systems using this arrangement of transducer array elements rely on mechanical motion of the probe to sweep an acoustic beam over a region of interest.
A second arrangement of transducer array elements comprises an array of transducer elements which is activated by electronic circuits which produce electronically induced time delays in the transducer element acoustic outputs. These time delays induce measurable phase delays, which cause the acoustic beam produced by the transducer element array to be steered and/or focused.
Links between electronic circuits which generate transmit pulses for transducer array elements and the transducer array elements that receive the transmit pulses are referred to as beamformer channels. Electronic steering and/or focusing of an acoustic beam produced by the transducer element array is achieved by electronically delaying transmit pulses, on a beamformer channel-by-beamformer channel basis, to create an effective protective cover having varying thickness.
Due to limits on: (a) the size and complexity of a cable connecting the ultrasound probe with the processing system and (b) the number of beamformer channels available in a reasonably priced ultrasound system, electronic focusing has been limited to a lateral direction (a direction parallel to the imaging plane). Focusing in an elevation direction (a direction perpendicular to the imaging plane) has been accomplished by placing a mechanical lens, of fixed curvature, on the probe face.
Conventional modifications in elevation focusing have been accomplished by changing the probe aperture and/or the properties of the mechanical lens. Although it is known that changing frequency can change focal depth (higher frequencies producing deeper focusing than lower frequencies), it is not considered advantageous to change frequency to change focal depth because higher frequencies are attenuated more rapidly in tissue than lower frequencies.
Consequently, it is known that in order to change elevation focusing of a transducer element array, one ought to change the elevation aperture and/or change the effective curvature of a lens associated with the transducer element array. For example, in imaging a deep organ, the lens ought to have a large aperture and mild curvature and, in imaging a shallower object, the lens ought to have a smaller aperture and a tighter curvature.
As is known, transducer array elements in an ultrasound probe can be arranged in a one-dimensional (1-D) array, a one-and-a-half-dimensional (1.5-D) array, or a two-dimensional (2-D) array (the size of a typical 1-D transducer array element is on the order of 0.5 wavelengths in the lateral direction and is on the order of 50 wavelengths in the elevation direction). In a 1-D array, transducer elements are generally disposed in the lateral direction, with a single row of elements in the elevation direction. Conventional phase linear arrays and curved arrays are generally considered 1-D transducer element arrays.
In a 1.5-D array, transducer elements are mounted in both the lateral and elevation directions, but control and data electrical connections are symmetrically connected about the elevation center so that an acoustic beam produced by a 1.5-D array can only be steered in the lateral direction. In a 2-D array, transducer elements are arranged in both the lateral and elevation directions, with electrical connections providing both transmit/receive control and excitation signals to transducer elements arranged in both directions. An acoustic beam produced by a 2-D array can be steered and focused in two dimensions. An example of a 2-D array ultrasound probe can be found in U.S. Pat. No. 5,186,175.
The advantages of 2-D array imaging are well known. For example, such advantages include the ability to electronically steer in two (2) dimensions (i.e., both lateral and elevation), enhanced resolution due to improved elevation focusing, and improved phase aberration correction through refined comparison of propagation velocities. The flexibility and enhanced resolution associated with 2-D transducers has eliminated the need for an acoustic lens shaped to mechanically focus the acoustic beams. However, the transducer elements still need to be protected. Consequently, the faces of 2-D transducers are configured with a relatively flat acoustically transparent material layer.
Sonographers can obtain images of a region within a body by properly positioning an ultrasound transducer against the body. In order to obtain images having diagnostic value, the sonographer may have to manipulate the position of the probe by sliding, rotating, and/or tilting the probe with respect to the patient.
A flat transducer face, such as those used with 2-D transducers, degrades image quality because it provides poorer contact with the body structures of a patient than a transducer with a curved surface. More specifically, a flat transducer surface causes spurious reflections and block portions of the acoustic aperture. Another disadvantage associated with a transducer configured with a flat face is that such transducers either have sharp edges, which can cause patient discomfort, or the transducers have an overly broad footprint to permit rounder edges.
Transducers configured with an overly broad footprint further impair contact between the transducer face and the patient, which can cause a sonographer to apply greater pressure along the longitudinal axis of the transducer in an attempt to improve contact between the transducer face and the patient. The increase in sonographer induced pressure can result in patient discomfort, as well as repetitive motion injuries to the sonographer. One area where maintaining appropriate contact between the transducer face and the patient is particularly problematic is intercostal cardiac and thoracic imaging. Generally, for these applications, the transducer housing contains a 2-D array of transducer elements selected for the expected enhanced resolution due to improved elevation focusing.
Consequently, there is a need for an improved transducer that addresses these and/or other shortcomings associated with conventional transducers.